bash 中带有变量、大括号和哈希字符的 ${0##...} 语法是什么意思？

What is the meaning of the ${0##...} syntax with variable, braces and hash character in bash?

提问by user215997

I just saw some code in bash that I didn't quite understand. Being the newbie bash scripter, I'm not sure what's going on.

我刚刚在 bash 中看到了一些我不太明白的代码。作为新手 bash 脚本编写者，我不确定发生了什么。

echo ${0##/*}
echo 
${string#substring}

I don't really see a difference in output in these two commands (prints the script name). Is that #just a comment? And what's with the /*. If it is a comment, how come it doesn't interfere with the closing }brace?

我真的没有看到这两个命令的输出差异（打印脚本名称）。这#只是评论吗？和/*. 如果是注释，它怎么不干扰}右大括号？

Can anyone give me some insight into this syntax?

任何人都可以让我深入了解这种语法吗？

回答by Mark Byers

See the section on Substring removalin the Advanced Bash-Scripting Guide?:

请参阅高级 Bash 脚本指南中的子字符串删除部分？：

${string##substring}

Deletes shortest match of substringfrom front of $string.

${string#substring}

Deletes longest match of substringfrom front of $string.

${string##substring}

删除substring从前面的最短匹配项$string。

${PARAMETER##WORD}  Results in removal of the longest matching pattern from the beginning rather than the shortest.
for example
[ian@pinguino ~]$ x="a1 b1 c2 d2"
[ian@pinguino ~]$ echo ${x#*1}
b1 c2 d2
[ian@pinguino ~]$ echo ${x##*1}
c2 d2
[ian@pinguino ~]$ echo ${x%1*}
a1 b
[ian@pinguino ~]$ echo ${x%%1*}
a
[ian@pinguino ~]$ echo ${x/1/3}
a3 b1 c2 d2
[ian@pinguino ~]$ echo ${x//1/3}
a3 b3 c2 d2
[ian@pinguino ~]$ echo ${x//?1/z3}
z3 z3 c2 d2

substring从 前面删除最长的匹配项$string。

The substring may include a wildcard *, matching everything. The expression ${0##/*}prints the value of $0unless it starts with a forward slash, in which case it prints nothing.

子字符串可能包含通配符*，匹配所有内容。该表达式${0##/*}打印 的值，$0除非它以正斜杠开头，在这种情况下它不打印任何内容。

_{? The guide, as of 3/7/2019, mistakenly claims that the match is of $substring, as if substringwas the name of a variable. It's not: substringis just a pattern.}

_{? 截至 2019 年 3 月 7 日，该指南错误地声称匹配项为$substring，就好像substring是变量的名称一样。它不是：substring只是一种模式。}

回答by Paul Creasey

Linux tip: Bash parameters and parameter expansions

Linux 技巧：Bash 参数和参数扩展

##代码##

回答by Ignacio Vazquez-Abrams

See the Parameter Expansionsection of the bash(1)man page.

请参阅手册页的Parameter Expansion部分bash(1)。